Part 2N: Field of View (FOV) and Telescopes

*Reminder for Teachers and Leaders: See Section 2: Parts K-R notes found in the Resources for IDATA.

Part 2K-R Notes and Answers


Think back to the model of the celestial sphere we used – the umbrella.

  • a) Predict (really, take a guess!) how big of an area on the inner surface of this umbrella, which is our model of the sky, the IDATA telescopes can take an image of. Use Prompt6.
  • b) In reality, the Prompt6 telescope in Chile has a field of view (FOV) of 15.1 arcminutes. What does this mean?

Think of a circle. It is 360 degrees from one point all the way around to the same point. A half circle has 180 degrees, like the dome of the umbrella, from one edge to the other edge. Let’s use the umbrella model to find out what an arcminute is:

  • 1) Divide each of the 180 degrees into sixty minutes.
  • 2) Then, if each degree contains 60 minutes, there are 180 X 60 total arcminutes in the half dome of the umbrella. This is a total of 10,800 arcminutes from horizon to horizon, or, using the umbrella, from one edge to the other.

We can use our umbrella model of a celestial sphere to get a good feel of what 15 arcminutes are. First, let’s try a few easy “hand” rules to get a feel for degrees:

1. If you hold your thumb out at arm’s length, the width of your thumb is one degree.

  • The moon only takes up ½ a degree of the sky – about the width of your smallest finger.
  • If the umbrella surface is about an arm’s length from the center, you can put your thumb on the umbrella surface to get an idea of one degree.

2. A closed fist represents 10 degrees. Try that on the umbrella surface as you stand at the center.

3. Recall, the Prompt6 telescope has a filed of view (FOV) of 15 arcminutes. This is half the width of your smallest finger.

  • Put your little finger against the inner surface of the umbrella.
  • A small bead, half the width of this finger, can be stuck to the inner surface of the umbrella to represent this field of view.

4. The Yerkes-24 has a Field of View of only 10 arcminutes, smaller than Prompt6’s.

5. The moon’s angular size of 30 arcminutes or 1/2 degree, can be represented by a velcro sticky dot anywhere on the inside surface of the umbrella.

***Try This:

  • At night, when the moon is present, hold your little finger out at arm’s length. It should easily cover the moon. Pretty neat.
  • The sun also has an angular size of 30 arcminutes.

Record in your journal your thoughts on the Field of View of the telescopes discussed.



The Yerkes-24 has a mirror with a diameter of 24 inches.

Draw a circle with this diameter on the board or a large piece of paper.

Prompt6 is 16 inches in diameter. The Yerkes-41 is 40 inches in diameter. (The Yerkes Telescopes are currently offline).

  • Measure these out also, either on paper, the whiteboard, the floor, or whatever is convenient. Make them tactile by cutting them out or tracing with a puffy ink pen, or anything that can be easily felt.
  • The size of these mirrors enable you to collect MUCH more light than your eye. The area of the sky the light is collected from. the Field of View (FOV), is small, that’s why the mirror has to be big!

The light, as ‘counts’, is collected in the pixels of the CCD camera and recorded in the computer.


We have been working with very small matrices to model the CCD camera:

  • a 3×3 matrix on the CCD poster, a 5×5 matrix with the egg crate, and various small matrices with the Quorum commands.
  • The 3×3 poster equals 9 pixels, the 5×5 egg crate equals 25 pixels, and your various matrices defined in Quorum also had small total pixel numbers.

The actual CCD camera on the Prompt6 is 2048×2048 = 4,194,304 pixels.

The camera on the Yerkes-24 is 1024×1024 = 1,048,576 pixels.


No wonder why it is easier to program a computer to do the work of analyzing the images.